Low tire warning system with axle torque signal

ABSTRACT

A system and method that monitors four wheel sensors of an automotive vehicle to determine changes in the effective rolling radii of any wheel and a low pressure tire condition. Certain vehicle operating conditions, such as excessive or very low speeds, driven wheel torque, braking, and turns, are determined from the sensor outputs and are used to prevent erroneous detection of changes in effective rolling radius. Data from the sensors that is determined to be acceptable is accumulated over time as displacement values for each wheel. When the displacement values exceed a predetermined value, a metric function is used to compare differences detected in accumulated displacement values for the front pair of wheels and for the rear pair of wheels. If the difference between compared pairs of wheels is excessive, as compared with a predetermined baseline metric value, a low tire condition is detected. Further accumulation of instances where the metric indicates a detected condition, provides confidence that the condition actually exists and a warning is provided to the vehicle operator. Resetting of the system and the baseline metric is performed when the low tire condition is corrected or when the tires are checked and properly inflated.

BACKGROUND OF THE INVENTION

This invention relates to the field of low tire warning systems for automotive vehicles and more specifically to a warning system that derives information for determining when any particular pneumatic tire is low by monitoring the angular rotation of each wheel and axle torque of the driven wheels.

Several low tire detection systems have been described in the literature, including those that monitor tire pressure and profile height. More recent systems have been described which utilize effective rolling radius calculations to determine when the radius of one of the wheels varies. The generally employed principle of using the effective rolling radius relies on the fact that a wheel with a flat or low pressure tire has an incrementally smaller effective rolling radius than a nominally inflated tire. Often, wheel displacement sensors are used to measure the angular displacement of each wheel. Each of these measurements are related to the effective rolling radius. In this context, the effective rolling radius is defined as the ratio of the true forward distance traveled by the center of a wheel divided by the angular displacement measured over this distance.

A problem with relying on the effective rolling radius of a radial construction tire is that its radius is only weakly dependent on tire pressure. The large "hoop tension" in the tire belt keeps the tire rolling radius almost constant with respect to tire inflation. For example, tests indicate that a tire inflated to only 5 psi will have a rolling radius approximately 0.9% smaller than if it were inflated to its nominal pressure, 30 psi. However, very accurate measurement of rolling radius has become economically feasible due to the enhanced dynamic range of modern 16-bit microprocessors commonly used in Anti-lock Braking Systems (ABS) and which read the wheel revolution sensors.

Another challenge in detecting low tire pressure is that some tire characteristics have a larger influence upon wheel effective rolling radius than inflation pressure. Tire-to-tire manufacturing tolerances typically vary the effective rolling radius by up to 1.2%. Also, during the tire break-in period, approximately the first 100 miles, effective rolling radius typically can change up to 0.6%. Tread wear also significantly changes the effective rolling radius over the tire lifetime, typically up to 3.6%.

Vehicle operating conditions will also cause significant changes to the effective rolling radius. These conditions are those which cause wheel slippage, those related to the use of a "space-saver" spare tire, and those related to speed. Generally speaking, any maneuver which causes even slight to moderate wheel slippage will cause the effective rolling radius to change by an amount greater than that to be caused by pressure variation alone. Such maneuvers include accelerating, decelerating using brakes, steering through sharp turns, and any combinations of these.

There also are other vehicle operating conditions which influence wheel effective rolling radius in a way not related to tire slippage. These include vehicle operation with a space-saver spare tire, and vehicle operation at very high or very low speeds. The smaller space-saver spare tire has an effective rolling radius that may differ significantly (e.g. 10%) from the other wheels. However, in some performance cars with large diameter brakes, corresponding large diameter wheels, and low profile tires, the use of a space saver spare wheel and tire which is narrow, may have essentially the same effective rolling radius as the other wheels. Differentiating this spare from the other wheels is very difficult in this situation, and the space-saver might be misinterpreted as a low tire.

Vehicle operation at very high speeds, such as those well above the U.S. national legal limit of 65 mph (105 kph), will cause high centrifugal forces in the wheels which can tension the perimeters of the tires in such a way that a low tire will take on the same or larger effective rolling radius as a nominally inflated tire.

Vehicle operation at very low speeds (e.g. less than 6 mph or 10 kph) also poses several problems for effective rolling radius based systems. One is the increased likelihood of wheel slip due to acceleration, deceleration, and steering. This is because low speed operation is not a sustained operating point, but a transitional one during which the car is decelerating to stop, accelerating to normal driving speeds, or steering through sharp turns and corners. Also at low speeds, the wheel rotation sensors' signals drop to a very low amplitude level and become noisy or non-existent. This loss of signal at low speed is a characteristic of current level wheel rotation sensor technology.

In U.S. Pat. No. 5,721,528 (Boesch et al.), assigned to the assignee of the present invention, a system and method was disclosed that monitors four wheel sensors of an automotive vehicle to determined changes in the effective rolling radii of any wheel and a low pressure tire condition.

SUMMARY OF THE INVENTION

The present invention is a new and improved low tire warning system employed on a four wheeled automotive vehicle to detect the occurrence of a flat or low tire inflation and to provide a warning to the vehicle operator. The invention utilizes four wheel displacement sensors already in place on vehicles that include ABS systems in addition to an axle torque signal from the powertrain control module for each of the driven wheels. A processor module also is employed and either may be a subcomponent of the ABS control module or a unique electronic module. A warning device notifies the vehicle operator when a low tire problem is detected, and a reset switching means is included, which allows the driver or tire mechanic to clear the low tire warning.

The principle of operation is based on the fact that a wheel with a flat or low pressure tire has an incrementally smaller effective rolling radius than a nominally inflated tire. In this invention, wheel displacement sensors measure the angular displacement of each wheel. That measurement is related to each wheel's effective rolling radius (i.e., the ratio of the true forward distance traveled by the center of the wheel divided by the angular displacement over this distance).

A novel aspect of the present invention is an algorithm which combines the measured wheel displacements in a way which indicates over time the variation of the wheels' effective rolling radii, which in turn reflects the tire pressure condition. Particular fore/aft vehicle asymmetries are taken into account by the algorithm to accommodate for vehicle front/rear weight differences, front/rear wheel torque differences, and front/rear tire differences and therefore avoid erroneous indications of a low tire condition.

Another novel aspect of the present invention, and an improvement over the aforementioned U.S. Pat. No. 5,721,528, is monitoring axle torque of the driven wheels to increase sensitivity to pressure and reduce the response time. In particular, when axle torque values are high, the axle is applying higher longitudinal forces to the tire contact patch on the ground, which causes the patch to slip. This slip occurs in steady-state conditions of climbing grades, pulling a trailer, or at higher speeds due to aerodynamic drag loads increasing exponentially with speed, but not slip due to transient slip due to low traction conditions. The torque signal is used to suspend measurement of wheel rotation data for low tire pressure warning when the axle torque exceeds a predetermined upper troque limit which indicates a steady-state slip condition as just described. When axle torque is below a predetermined lower torque limit, wheel rotation measurement is also suspended.

The algorithm is enabled when the vehicle operates above a minimum speed and below a maximum speed, above a minimum torque and below a maximum torque, and below maximum absolute values of both longitudinal and lateral accelerations. This prevents the tire warning system from considering measurements taken when the tires may be slipping on the roadway or subjected to the high and low speed characteristics mentioned above. Unlike some prior art algorithms proposed for flat or low tire warning based upon wheel revolutions, this algorithm is enabled during vehicle steering, as long as the other conditions on speed, acceleration and torque are met.

In the present invention, the methodology partitions the vehicle fore/aft in order to make an estimation of true vehicle forward progress (i.e., the front forward progress and rear forward progress are separately estimated). This approach overcomes the effect of the following three attributes that otherwise may influence accurate determinations of low tire occurrences:

1. The vehicle weight distribution is typically more consistent side-to-side than front-to-rear. A fore/aft partitioning tends to reduce the effect of front/rear weight differences on effective rolling radius. This makes the metric calculation more reliable.

2. In front wheel drive and rear wheel drive vehicles, longitudinal tire forces (acceleration and braking) will affect the front and rear true forward progress estimates differently. Partitioning makes the calculated metric more immune to the effects of longitudinal tire forces.

3. It is more likely that tires with different characteristics (new or from a different manufacturer or of different sizes) will be used on the front or rear of the vehicle rather than from side-to-side. Therefore, fore/aft partitioning tends to group tires with like characteristics.

The described method uses the estimated true front forward progress and true rear forward progress to generate a low tire signal, called "the metric". The metric is compared to a predetermined "baseline" metric value which represents the particular car's tire-to-tire differences not related to pressure. If the difference between current metric value and baseline metric value is larger than a threshold, which is fine-tuned for this vehicle type and tire type, at least one tire is perceived to be low.

Because warning the driver about a low tire is distracting, the algorithm first builds confidence by requiring that a low tire is repeatably indicated by the metric. For this purpose, a "confidence filter" is provided. The confidence filter output is a single number which represents the consistency of a collection of repeated low tire metric evaluations. The confidence number increases each time that the metric indicates low, and decreases each time that it indicates no low tire problems. When the confidence number exceeds a predetermined threshold, a signal is generated and the driver is warned. The amount by which the confidence filter is incremented may differ from those by which it is decremented. The fine-tuning of the increments and decrements for a vehicle type and tire type allow a trade-off between low tire warning response time and low tire warning accuracy. Also, the increment and decrement size can be made dependent on vehicle operating conditions such as speed, acceleration, road roughness, steering, etc., as perceived by the wheel rotations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall architecture diagram showing the hardware used for the low tire warning system of the present invention.

FIG. 2 is an abstract representation of essential data flows in the low tire warning processor.

FIG. 3A and FIG. 3B constitute a detailed flow diagram for the preferred embodiment of the algorithm employed in the low tire warning processor.

FIG. 4 is a detailed flow diagram of the low tire confidence determination step shown in FIG. 3B.

FIG. 5 is a graph depicting a time history of axle torque showing upper and lower torque limits which, when measured axle torque is therebetween, enables the algorithm employed in the low tire warning processor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A typical hardware arrangement is shown in FIG. 1. Here, the right front, the right rear, the left front and left rear wheels are represented as 12, 14, 16 and 18, respectively. The rotation of those wheels are sensed by corresponding angular displacement sensors 22, 24, 26 and 28. Each sensor outputs a variable frequency signal which is directly related to the rotational speed of the wheel and the number of teeth (typically 50) on the sensor. The rotation signals are then input to a low tire warning processor 100. A powertrain control module 11 (PCM) calculates axle torque for each of the driven wheels and provides a signal to the low tire warning processor 100. Processor 100 repetitively executes an algorithm which evaluates the wheel rotation signals and axle torque, and provides a signal to activate a warning indicator 30 to alert the driver when a low tire is detected. A warning reset switch 40 is also represented to deactivate the warning indicator, after service is made for the low tire problem.

FIG. 2 presents a high level description of the function of the algorithm which is executed within the low tire warning processor 100 of FIG. 1, and showing essential data flows of the algorithm.

The wheel speed pulses are input at step 101 to a "process wheel pulses" process 102. The zero crossings of these pulses are monitored over relatively short sampling periods to derive sampling velocity values for each wheel. Those velocity values are then filtered over relatively longer predetermined sampling period to provide estimates of individual wheel velocities. The estimated wheel velocities are further filtered to provide estimates of individual longitudinal wheel accelerations, as well as lateral accelerations for the front pair of wheels and for the rear pair of wheels.

The velocity values and acceleration values are compared against predetermined levels to determine if they are acceptable for further processing, in the "data quality check" process 104, as are the estimated wheel torque signals 107 from the PCM 105. Process 104 determines if the operating conditions of the vehicle are suitable for the system to accept the velocity, acceleration, and torque values. If accepted, a "data valid" signal enables an "accumulate wheel displacement" process 106. When enabled, process 106 accepts wheel velocity values for each wheel, determines displacement values and accumulates wheel displacement values until a predetermined distance is determined to have been traveled by any wheel. After collecting valid wheel velocity data over a required distance, the total wheel displacement values are each passed to the "low tire pressure sensing metric" process 108, and the accumulators used in process 106 are zeroed (not shown).

The low tire pressure sensing metric process 108 utilizes a unique algorithm which subtracts the difference of the accumulated displacement values between the front wheels divided by their mean displacement from the accumulated displacement values between the rear wheels divided by their mean displacement. This processing of the individual wheel displacements determines if a low tire pressure condition exists for a tire. The calculated metric is then compared with a predetermined baseline metric to determine an absolute value of a calibrated metric. The absolute value of the calibrated metric is then processed in a "low tire warning confidence filter" process 110.

In process 110, the absolute value of the calibrated metric is compared with a predetermined threshold value which is determined as an acceptable range of values equally above and below the baseline value. This is based on the presumption that the metric must differ sufficiently from the established baseline metric to confirm that the metric value indeed indicates a low tire condition. If the calibrated metric does exceed the predetermined threshold, a low tire condition is determined. That occurrence is then accumulated until there is a sufficient number of such occurrences to provide confidence that a low tire condition has been consistently detected and a warning should be given at process 112.

In process 112, a warning indicator notifies the driver to check the tires. At that time, the operator or the service technician should rest the system so that the several accumulations may be zeroed and the baseline updated according to a filtering process that reflects the history of earlier estimations.

It should be noted that while the sensing metric process of FIG. 2 is described in such a way as to sense a single low tire, it may also be capable of sensing two diagonal low tires, any combination of 3 low tires, and any combination of two low tires as long as the tires do not lose pressure at exactly the same rate. In those instances, the higher the metric's absolute value, the more likely there is that a low tire condition exists.

FIGS. 3A and 3B constitute a flowchart which presents a more detailed description of the process which is executed within the low tire warning processor 100 of FIG. 1, and which is summarized in the above description of FIG. 2. (Although not shown, it should be noted that a conventional counter/timer processing unit can be used to process the individual wheel rotation signals from the angular rotation sensors 22, 24, 26 and 28 shown in FIG. 1 and gathered over a relatively short time period. Rotations are sensed by using conventional ABS sensors with "tone rings" emitting typically 100 signal transitions per wheel revolution. In this case, a relatively short time period of 7 msec was selected. Such a processing unit should generate an accumulated zero-crossings signal for each wheel, and an elapsed time signal. This information is in turn gathered by the process of FIG. 3A approximately every 7 msec.)

Following the start 200, a relatively short time period of 7 msec. is established at loop step 210 in which to sample rotational zero-crossing accumulations and generate unfiltered wheel velocity values in step 220. Step 220 can be performed in any conventional manner.

The four unfiltered wheel velocity values are then processed at step 230 every 7 msec. using a digital low pass filtering and scaling technique according to the relationship:

    Vk=α(Vk-1)+(1-α)(μk),

wherein

Vk becomes the filtered velocity value for the most recent of k samples (in this case 7 samples in any 49 msec. sampling period);

α is a constant having a value that is less than 1;

Vk-1 is the filtered velocity value for the sampling immediately preceding the most recent sampling; and

μk is the sensed angular rotation rate for the most recent 7 msec. sampling of the several that occur during the predetermined sampling period, selected as 49 msec.

This filtering technique produces a smoothed approximation value for each wheel and is termed as filtered wheel velocity values V_(w) for each wheel. (i.e., V_(wfl) is the filtered wheel velocity value calculated for the front left wheel; V_(wfr) is for the front right wheel; V_(wrl) is for the rear left wheel; and V_(wrr) is for the rear right wheel.)

In step 240, a high pass filtering technique is used every 7 msec. to generate four longitudinal wheel acceleration signals A_(lng) according to the relationship:

    A.sub.lng k=δ(A.sub.lng k-1)+β[Vk-Vk-1],

wherein

δ is a constant, less than 1;

β is a constant;

Vk is the most recently calculated filtered velocity;

Vk-1 is the calculated filtered velocity for the sampling imamediately preceding the most recent sampling; and A_(lng) k-1 is the longitudinal acceleration derived from the calculated filtered velocity immediately preceding the most recently calculated velocity.

The steps performed at 220, 230 and 240 are repeated until 49 msec has been determined as expired in step 250. When this predetermined sampling period has expired, the filtered velocities and longitudinal acceleration values for each wheel are sampled at step 260 as V_(w) and A_(lng) values for each of the four wheels.

In step 270, a front lateral acceleration value A_(latf) is generated for the front pair of wheels. In step 280, a rear lateral acceleration value A_(latr) is generated for the rear pair of wheels. For each pair of wheels the respective steps 270 and 280 are performed by using a filter technique according to the relationships:

    A.sub.latf =κ(V.sub.wfl -V.sub.wfr)(V.sub.wfl +V.sub.wfr) and A.sub.latr =κ(V.sub.wrl -V.sub.wrr)(V.sub.wrl +V.sub.wrr),

wherein

κ is a constant;

V_(wfl), V_(wfr), V_(wrl) and V_(wrr) are as described above.

After the lateral acceleration values have been generated, all the velocity and acceleration values are processed for data quality in steps 290-330 (FIG. 3B).

In step 290, each sampled velocity value V_(wfl), V_(wfr), V_(wrl), and V_(wrr) is compared with a predetermined minimum acceptable velocity value V_(min) to determine if the sampled velocity is at an acceptable level. In this embodiment, V_(min) is selected as approximately 5 mph or 8 kph. If any velocity value is below the minimum acceptable velocity value, the data is rejected and further processing with that data is prevented. However, if all four of the sampled velocity values are higher than the minimum acceptable value, then the measured axle torque for each of the driven wheels is compared to a predetermined upper torque limit, T_(max), in step 305. If any torque value is above the maximum acceptable torque value, the data is rejected and further processing with that data is prevented. However, if all measured torque values of the driven wheels are lower than the maximum acceptable value, all four of the sampled velocity values are then compared against a predetermined maximum acceptable velocity value V_(max) in step 310. In this embodiment, V_(max) is selected as approximately 65 mph (104 kph). If any velocity value is greater than the maximum acceptable velocity value, the data is rejected as unreliable because of the factors discussed above and further processing with that data is prevented. However, if all four of the sampled velocity values are lower than the maximum acceptable value, then the measured axle torque for each of the driven wheels is compared to a predetermined lower torque limit, T_(min), in step 315. If any torque value is below the minimum acceptable torque value, the data is rejected and further processing with that data is prevented.

In step 320, all four of the longitudinal acceleration values A_(lng) are compared with a predetermined maximum acceptable longitudinal acceleration value A_(lngmax). If any of the longitudinal acceleration values A_(lng) are greater than the predetermined maximum acceptable longitudinal acceleration value A_(lngmax), further processing of that data is prevented. This allows further processing only if there is no excessive longitudinal acceleration detected in any wheel, that may be due to braking, slipping or rapid application of wheel torque, as discussed above.

If the longitudinal acceleration data is acceptable, the lowest lateral acceleration value A_(lat) of those generated in step 280 is compared with a predetermined maximum acceptable lateral acceleration value A_(latmax). The lowest value of A_(lat) is selected for comparison because the occurrence of a low pressure tire will result in one of the two values of A_(lat) to be higher than the other one. Therefore, in order to ensure that data indicative of a low tire will be further processed, it is prudent to validate both values of A_(lat).

Once the data has been validated, the estimated displacement θ accumulation for each wheel is compared with a predetermined displacement value θ_(p). When the accumulated displacement for each wheel exceeds θ_(p), a metric is calculated in step 360. However, if the accumulated displacement value for any wheel is below θ_(p), the θ for each wheel is updated in step 350 and more data is collected. In step 350, the displacement values are updated according to the relationship of:

θ=∫V_(w) (dt), for each wheel.

In step 360, the metric "U" is calculated according to the relationship:

    U=((θfl-θfr)/θflθfr)-((θrl-θrr)/θrlθrr),

wherein

θfl is the front left wheel accumulated displacement value,

θfr is the front right wheel accumulated displacement value,

θflθfr is the mean value of θfl and θfr,

θrl is the rear left wheel accumulated displacement value,

θrr is the rear right wheel accumulated displacement value, and

θrlθrr is the mean value of θrl and θrr.

In the metric U, a difference between the displacement values of the paired wheels, is a direct reflection of the difference between effective rolling radii of those wheels and can be attributed to one tire having changed because of loss of air pressure. The metric is selected to accentuate the effect caused by the low pressure tire, by comparing the differences in displacements in each pair of wheels, front and back. If the difference in the effective rolling radii remains unchanged, then the calculated metric will remain low and be of no significance.

After the metric U is calculated, the displacement accumulators are reset to zero in step 370.

In step 380, the process looks to see if a reset signal has been received. As discussed above, such a signal would be provided following service to correct a previously detected low tire condition. In addition, the reset signal may be provided as part of a regular maintenance routine in which all the tires are checked and provided with proper inflation. If such a reset signal were detected at step 380, a tire warning reset routine 390 would be performed to update or recalculate a baseline metric U₀ from future metric measurements.

The tire warning reset routine 390 updates a baseline metric value U₀ when the tire pressure conditions are thought to be acceptable to the driver or tire mechanic. A reset is requested by activating the warning reset switch 40 in FIG. 1. Although the baseline metric U₀ will not be updated immediately, the processor 100 will immediately inhibit the low tire warning indicator 30.

The baseline value U₀ is based upon an average value of the tire warning metric U. The particular averaging means may be any of the following: a batch average for a chosen fixed number of metric U updates; a low pass filtered value of metric U updates taken over a fixed chosen time interval; or a low pass filtered value of metric U updates taken over a self-adapting time interval, with the interval period automatically adjusted until a performance criteria on the maximum acceptable variance of U₀ is met. The parameters of the selected averaging method are fine-tuned for the particular vehicle and tire combination.

In step 400, the calculated metric U is compared with the predetermined baseline metric U₀ to derive an absolute value of the difference. This value is referred to as the calibrated metric 1U^(cal) 1. The calibrated metric is then processed in a low tire warning confidence routine 410, shown in FIG. 4. If the confidence level is determined in routine 410 to be sufficiently high, the low tire warning will be activated in step 420.

In FIG. 4, the routine is shown for determining if the calibrated metric 1U_(cal) 1 is sufficient to indicate that a low tire condition is detected, and when a sufficient number of such detections occur to provide assurance that a low tire condition actually exists.

An accumulation of sufficiently high values of 1U_(cal) 1 is made in routine 410 and are represented by a confidence factor F_(o). At step 411, F_(o) is read. In step 412, the calibrated metric 1U_(cal) 1 is compared with a predetermined calibrated metric threshold value U_(thresh). If the comparison in step 412 determines that 1U_(cal) 1 exceeds U_(thresh), an updated confidence factor F_(n) is calculated in step 415 by incrementing the confidence factor F_(o) read in step 411 by a predetermined increment factor M. In this case, M is and an integer, but may be a constant, or a variable based on conditions selected by one who implements the invention. After step 415, confidence factor F_(o) is set equal to updated confidence factor F_(n) in step 417.

In step 416, the updated confidence factor F_(n) is determined by decrementing the confidence factor F_(o) by a predetermined decrement factor D, when the calibrated metric 1U_(cal) is determined in step 416 to be below the calibrated metric threshold value U_(thresh). In this case, D is and an integer, but may be a constant, or a variable based on conditions selected by one who implements the invention. In this embodiment, the use of a decrementing step to offset the incrementing step, means that the building of a confidence factor to a predetermined threshold value may take longer, but the confidence in the determination of a low tire condition will be stronger and less likely to give false warnings.

As an alternative to the count down of the confidence factor F_(n) in step 416, that step can be eliminated in favor of step 415 alone. If the factor M is carefully determined, the appropriate confidence level can be reached.

In step 418, the confidence factor F_(o) from step 417 is compared with a predetermined confidence threshold to determine if the system has sufficient confidence to provide a warning to the vehicle operator that a low tire condition exists and that service should be performed to correct the condition. 

We claim:
 1. A low tire inflation warning system for an automotive vehicle comprising:a wheel containing a pneumatic tire on both sides of the front of the vehicle; a wheel containing a pneumatic tire on both sides of the rear of the vehicle; angular displacement sensors associated with each wheel, to provide output signals corresponding to the rotation of each wheel; processor means for sampling said sensor signals over predetermined time periods to determine the occurrence of a low tire; and warning means activated when said determining means indicates the occurrence of a low tire greater than a predetermined threshold number of occurrences; said processor means is programmed to:determine a velocity value for each wheel, a longitudinal acceleration value for each wheel, a lateral acceleration value for the front pair of wheels, a lateral acceleration value for the rear pair of wheels, a displacement value for each wheel, a torque value for each driven wheel, and a metric value based upon a predetermined relationship of an accumulation of displacement values for each wheel during several sampling periods; prevent the determining of the displacement of each wheel when any velocity value or any torque value is outside upper and lower predetermined limits; prevent the determining of the displacement of each wheel when any longitudinal acceleration value is above a predetermined maximum limit; prevent the determining of the displacement of each wheel when the lowest of the two said lateral acceleration values is above a predetermined maximum limit; and prevent the determining of the metric value until the accumulation of displacement values for any wheel exceeds a predetermined threshold displacement value.
 2. A low tire inflation warning system as in claim 1, wherein said processor means is further programmed to compare each determined metric value with a predetermined base-line metric value and to accumulate each occurrence that said comparison results in said metric differs from said predetermined base-line metric value by a at least a predetermined amount; and to provide said activation signal to said warning means when said accumulated occurrences exceed a second predetermined value.
 3. A method of detecting a partially deflated pneumatic tire on a vehicle having a wheel at both sides of the front of the vehicle and a wheel at both sides of the rear of the vehicle, comprising the steps of:sensing the angular rotation of each wheel; calculating the velocity of each wheel according to the sensed angular rotation over a predetermined sampling period; sensing an axle torque for each driven wheel deriving the longitudinal acceleration of each wheel over said sampling period; comparing each velocity value with a predetermined minimum acceptable velocity value and a predetermined maximum acceptable velocity value to determine if any velocity value is below the predetermined minimum velocity value or above the predetermined maximum velocity value; comparing each axle torque value with a predetermined minimum acceptable torque value and a predetermined maximum acceptable torque value to determine if any torque value is below the predetermined minimum torque value or above the predetermined maximum torque value; comparing each longitudinal acceleration value with a corresponding predetermined maximum acceptable longitudinal acceleration value to determine if any longitudinal acceleration value is above the predetermined maximum longitudinal acceleration value; calculating and accumulating displacement values θ for each wheel over several of said sampling periods in which said comparing steps indicate velocity values and acceleration values are within acceptable limits; calculating a metric U=((θfl-θfr)/θflθfr)-((θrl-θrr)/.theta.rlθrr) when said θ for any wheel exceeds a predetermined displacement value, whereinθfl is the front left wheel accumulated displacement value, θfr is the front right wheel accumulated displacement value, θflθfr is the mean value of θfl and θfr, θrl is the rear left wheel accumulated displacement value, θrr is the rear right wheel accumulated displacement value, θrlθrr is the mean value of θrl and θrr; comparing said calculated metric with a first predetermined metric value; accumulating each occurrence that said comparing step results in said calculated metric exceeding said first predetermined metric value by at least a predetermined amount; and providing an activation signal to a warning device when said accumulated occurrences exceed a predetermined occurrence value.
 4. A low tire inflation warning system as in claim 3, wherein said accumulating step includes positively accumulating each occurrence that said comparing step results in said calculated metric exceeding said first predetermined metric value and negatively accumulating each occurrence that said comparing step results in said calculated metric not exceeding said first predetermined metric value; and providing an activation signal to said warning means when the sum of said positively and negatively accumulated occurrences exceeds said second predetermined value.
 5. A method as in claim 3, wherein said step of calculating the velocity value V of each wheel is performed by using a filter technique according to the relationship:

    Vk=α(Vk-1)+(1-α)(μk),

wherein Vk becomes the filtered velocity value for the most recent of k samples; α is a constant, less than 1; Vk-1 is the filtered velocity value for the sampling immediately preceding the most recent sampling; and μk is the sensed angular rotation rate for the most recent sampling of several that occur during the predetermined sampling period.
 6. A method as in claim 5, further including the steps of:deriving the lateral acceleration of the front pair of wheels over said sampling period; and deriving the lateral acceleration of the rear pair of wheels over said sampling period; and wherein said step of deriving the lateral acceleration A_(lat) of each said pair of wheels is performed by using the filter technique according to the relationships:

    A.sub.latf =κ(V.sub.wfl -V.sub.wfr)(V.sub.wfl +V.sub.wfr) and A.sub.latr =κ(V.sub.wrl -V.sub.wrr)(V.sub.wrl +V.sub.wrr),

wherein κ is a constant; V_(wfl) is the velocity value calculated for the front left wheel; V_(wfr) is the velocity value calculated for the front right wheel; V_(wrl) is the velocity value calculated for the rear left wheel; and V_(wrr) is the velocity value calculated for the rear right wheel.
 7. A method as in claim 3, further including the steps of:deriving the lateral acceleration of the front pair of wheels over said sampling period; and deriving the lateral acceleration of the rear pair of wheels over said sampling period; and wherein said step of deriving the lateral acceleration A_(lat) of each said pair of wheels is performed by using the filter technique according to the relationships:

    A.sub.latf =κ(V.sub.wfl -V.sub.wfr)(V.sub.wfl +V.sub.wfr) and A.sub.latr =κ(V.sub.wrl -V.sub.wrr)(V.sub.wrl +V.sub.wrr),

wherein κ is a constant; V_(wfl) is the velocity value calculated for the front left wheel; V_(wfr) is the velocity value calculated for the front right wheel; V_(wrl) is the velocity value calculated for the rear left wheel; and V_(wrr) is the velocity value calculated for the rear right wheel.
 8. A method as in claim 3, wherein said step of deriving the longitudinal acceleration A_(lng) of each wheel is performed by using the filter technique according to the relationship:

    A.sub.lng k=α(A.sub.lng k-1)+β[Vk-Vk-1],

wherein α is a constant, less than 1; β is a constant; Vk is the most recently calculated velocity for the most recent of k samplings of sensed angular rotation rate of several samplings that occur during the predetermined sampling period; Vk-1 is the calculated velocity for the sampling of sensed angular rotation rate immediately preceding the most recent sampling; and A_(lng) k-1 is the longitudinal acceleration derived from the calculated velocity immediately preceding the most recently calculated velocity.
 9. A method as in claim 3, further including the step of comparing the lowest value of the derived lateral acceleration of the front pair and rear pair of wheels with a predetermined maximum acceptable lateral acceleration value prior to said step of calculating and accumulating displacement values.
 10. A method of detecting a partially deflated pneumatic tire on a vehicle having a wheel at both sides of the front of the vehicle and a wheel at both sides of the rear of the vehicle, comprising the steps of:sensing the rotation of each wheel and the axle torque of each driven wheel; sampling said sensed rotation of each wheel and said axle torque of the driven wheels over predetermined time periods to determine the occurrence of a low tire; and warning the vehicle operator when said step of determining indicates the occurrence of a low tire over a predetermined threshold number of occurrences; and said sampling includes the steps of; determining a velocity value for each wheel, a torque value for each driven wheel, a longitudinal acceleration value for each wheel, a lateral acceleration value for the front pair of wheels, a lateral acceleration value for the rear pair of wheels, a displacement value for each wheel, and a metric value based upon a predetermined relationship of an accumulation of displacement values for each wheel during several sampling periods; preventing the determining of the displacement of each wheel when any velocity value is outside upper and lower predetermined velocity limits; preventing the determining of the displacement of each wheel when any torque value is outside upper and lower predetermined torque limits; preventing the determining of the displacement of each wheel when any longitudinal acceleration value is above a predetermined maximum limit; preventing the determining of the displacement of each wheel when the lowest lateral acceleration value is above a predetermined maximum limit; and preventing the determining of the metric value until the accumulation of displacement values for any wheel exceeds a predetermined threshold displacement value.
 11. A method as in claim 10, wherein said sampling step further includes the steps of comparing each determined metric value with a predetermined base-line metric value and accumulating each occurrence that said comparing results in said metric differ from said predetermined base-line metric value by at least a predetermined amount; and to provide said warning when said accumulated occurrences exceed a predetermined threshold number of occurrences. 